Copyright and patents are well known mechanisms to protect the work of an author. In most cases copyright is used to protect art from being reproduced, whereas patents are used to protect inventions from being used with insufficient appreciation. The motivation in both cases is the establishment of possibilities to earn money with artistic or technical work. 
Since software can be understood both as artistic and technical, the strategies of protecting software are quite difficult.  
The problem gets even more complicated if the software developers does not have the motivation to earn money by selling the software.

In the summer term 2012 we attended the course \emph{Open Source and Intellectual Property in the Digital Society} held by Mirko Böhm. During the lectures we discussed the impact of open source and peer production in our economy and society. In this (term) paper we want to use our learned knowledge to provide information an key ideas behind the protection of free software. Therefore we give a brief introduction into copyright law to explain how copyleft licenses work. After that we present three projects who uses copyleft licenses to protect their work and analyze common strategies of these actors.